Add gl-badge for badges in milestone drawer

What does this MR do?

Add gl-badge for badges in milestone drawer

As a part of https://gitlab.com/gitlab-com/marketing/community-relations/contributor-program/hackathon/-/issues/50

Example Page: %Sales Quick Start - May 2020

Screenshots (strongly suggested)

Before	After

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

app/views/shared/milestones/_sidebar.html.haml

@@ -79,7 +79,7 @@
           %span= milestone.issues_visible_to_user(current_user).count
         .title.hide-collapsed
           = s_('MilestoneSidebar|Issues')
-          %span.badge.badge-pill= milestone.issues_visible_to_user(current_user).count
+          %span.badge.badge-muted.badge-pill.gl-badge.sm= milestone.issues_visible_to_user(current_user).count
           - if show_new_issue_link?(project)
             = link_to new_project_issue_path(project, issue: { milestone_id: milestone.id }), class: "float-right", title: s_('MilestoneSidebar|New Issue') do
               = s_('MilestoneSidebar|New issue')
@@ -110,7 +110,7 @@
           %span= milestone.merge_requests.count
         .title.hide-collapsed
           = s_('MilestoneSidebar|Merge requests')
-          %span.badge.badge-pill= milestone.merge_requests.count
+          %span.badge.badge-muted.badge-pill.gl-badge.sm= milestone.merge_requests.count
         .value.hide-collapsed.bold
           - if !project || can?(current_user, :read_merge_request, project)
             %span.milestone-stat